A method for improved engine operation

Abstract

 An method for starting and operating an engine 11 of a mild hybrid vehicle 5 following start-up is disclosed for an engine 11 having low and high voltage starting devices 13 and 14 controlled by an electronic controller 10. The method comprises using the high voltage starting device 14 as a motor to crank the engine 11 whenever possible so as to partially discharge a high voltage battery 18 and then using the high voltage starting device 14 as a generator after starting the engine 11 if the exhaust gas temperature needs to be increased so as to assist with light-off of one or more exhaust gas aftertreatment devices 6.

Description

[0001] This invention relates to an internal combustion engine and in particular to a method and apparatus for improved operation of such an engine.

[0002] It is well known to use a low voltage (12 volt) starter motor to start an internal combustion engine even when the engine is at a very low temperature such as for example minus 30°C. It is further known to provide an engine with a high voltage (for example 48 volt) belt integrated starter generator (BISG) that is driveably connected to a crankshaft of the engine by a drive belt and is able to be either driven by the engine to generate electrical power or drive the engine either to assist with power output or for starting the engine.

[0003] Normally the BISG is only used when the temperature of the engine is above a relatively warm temperature corresponding to a low end of a normal temperature operating range of the engine such as, for example 60°C, because a conventional BISG will normally have insufficient torque to start the engine at lower temperatures.

[0004] It is further known that the fuel saving ability of a vehicle is restricted or limited by the need to first light-off any exhaust aftertreatment devices. It is therefore desirable, particularly when the engine is cold, to apply a load to the engine after it has started in order to increase the temperature of the exhaust gas and thereby reduce the time taken to light-off any attached exhaust aftertreatment devices.

[0005] The BISG can be usefully employed to apply such a heating load but is often not able to do so because the current state of charge (SOC) of the associated high voltage battery will not permit the BISG to be run as a generator.

[0006] The inventors have realised that if the BISG is used over a larger temperature range to start the engine then the discharging effect of using the BISG to start the engine can beneficially be used to reduce the state of charge of the high voltage battery thereby allowing the BISG to be subsequently operated as a generator after the engine has started to apply a load to the engine and speed up light-off of the exhaust aftertreatment devices.

[0007] In addition, in the case of a mild hybrid vehicle, stop-start operation does not have to be restricted until the engine temperature has reached the low end of the normal temperature operating range of the engine (60 to 120°C) but can be used whenever BISG starting is possible thereby increasing the opportunities for stop-start operation.

[0008] It is an object of the invention to provide a method for improved engine operation.

[0009] According to a first aspect of the invention there is provided a method for improved engine operation comprising starting an engine of a motor vehicle having a high voltage belt integrated starter-generator and a low voltage starter motor, the high voltage belt integrated starter-generator being used whenever it is able to effectively crank the engine and, if after starting of the engine, it is required to light-off one or more exhaust aftertreatment devices, using the high voltage belt integrated starter-generator as a generator so as to load the engine to increase the temperature of the exhaust gas from the engine.

[0010] The method may further comprise using the high voltage belt integrated starter-generator as a generator to increase the temperature of the exhaust gas of the engine until it is no longer required to further increase the temperature of the exhaust gas.

[0011] The method may further comprise using the high voltage belt integrated starter-generator as a generator only if the state of charge of a battery charged by the high voltage belt integrated starter-generator is below a predefined upper charging limit.

[0012] If after starting of the engine and provided there is no requirement to light-off one or more exhaust aftertreatment devices, the method may further comprise determining whether torque assist of the engine is required and, if torque assist is required, using the high voltage belt integrated starter-generator as a motor to provide torque assist to the engine.

[0013] The method may further comprise using the high voltage belt integrated starter-generator for torque assist only if the state of charge of a battery providing power to the high voltage belt integrated starter-generator to operate it as a motor is above a minimum permitted charge level.

[0014] The high voltage belt integrated starter-generator may be able to effectively crank the engine unless the temperature of the engine is below a predefined low temperature limit below which the engine is started using the low voltage starter motor.

[0015] The low temperature limit may be a temperature well below the normal operating range of the engine.

[0016] The low temperature limit may be a temperature below zero degrees Celsius.

[0017] The low temperature limit may be a temperature falling within a range of minus 40 to plus 5 degrees Celsius.

[0018] The high voltage belt integrated starter-generator may be able to effectively crank the engine if it is able to crank the engine at a speed greater than a defined rotational speed.

[0019] The high voltage belt integrated starter-generator may be able to effectively crank the engine if it is able to produce an acceleration of the engine greater than a defined acceleration.

[0020] The high voltage belt integrated starter-generator may be able to effectively crank the engine if the state of charge of a battery used to power the high voltage belt integrated starter-generator is above a predefined level before cranking commences.

[0021] According to a second aspect of the invention there is provided a mild hybrid motor vehicle having an apparatus for improved engine operation comprising a high voltage belt integrated starter-generator, a low voltage starter motor and an electronic controller to control the operation of at least the high voltage belt integrated starter generator and the low voltage starter motor wherein the electronic controller is operable to use the high voltage belt integrated starter-generator to crank the engine whenever it is able to effectively crank the engine and, if after starting of the engine, it is required to light-off one or more exhaust aftertreatment devices of the motor vehicle, using the high voltage belt integrated starter-generator as a generator so as to load the engine to increase the temperature of the exhaust gas from the engine..

[0022] The high voltage belt integrated starter-generator may be able to effectively crank the engine unless the temperature of the engine is below a predefined low temperature limit below which the engine is started using the low voltage starter motor.

[0023] The low temperature limit may be a temperature well below the normal operating range of the engine.

[0024] The low temperature limit may be a temperature below zero degrees Celsius.

[0025] The low temperature limit may be a temperature falling within a range of minus 40 to plus 5 degrees Celsius.

[0026] The high voltage belt integrated starter-generator may be able to effectively crank the engine if it is able to crank the engine at a speed greater than a defined rotational speed.

[0027] The high voltage belt integrated starter-generator may be able to effectively crank the engine if it is able to produce an acceleration of the engine greater than a defined acceleration.

[0028] The high voltage belt integrated starter-generator may be able to effectively crank the engine if the state of charge of a battery used to power the high voltage belt integrated starter-generator is above a predefined level before cranking commences.

[0029] The invention will now be described by way of example with reference to the accompanying drawing of which:-

Fig.1 is a block diagram of a mild hybrid motor vehicle including an apparatus for starting and operating an engine of the motor vehicle in accordance with a second aspect of the invention;

Fig.2 is a flow chart of a method for starting an engine forming a first part of a method for improved engine operation in accordance with a first aspect of the invention; and

Fig.3 is a flow chart forming a second part of the method for improved engine operation in accordance with the first aspect of the invention.

[0030] With reference to Fig.1 there is shown a mild hybrid vehicle 5 having an engine system including an engine 11 drivingly connected to a gearbox 12 and an apparatus for starting the engine 11. One or more exhaust gas aftertreatment devices 6 are arranged to receive exhaust gas from the engine 11.

[0031] The apparatus for starting the engine 11 comprises an electronic controller in the form of a control unit 10, a low voltage starter system including a starter motor 13, a low voltage battery 17 and a low voltage battery management system 15 and a high voltage starter system including a belt integrated starter-generator 14, a high voltage battery 18 and a high voltage battery management system 16.

[0032] The apparatus for starting the engine 11 further comprises a DC to DC voltage converter for selectively connecting the high voltage battery 18 to the low voltage battery 17 for the purpose of recharging the low voltage battery 17 and a number of inputs 20 for providing information to the electronic controller 10.

[0033] A 'mild hybrid vehicle' is a vehicle having an electric motor/generator (starter-generator) driveably connected to an engine of the vehicle to:-

a/ assist the engine of the vehicle by producing mechanical torque using electricity stored in a high voltage battery (torque assist);

b/ capture energy from the vehicle with no fuel penalty;

c/ store captured energy as electricity in the high voltage battery;

d/ start the combustion engine of the vehicle; and

e/ provide electrical energy to users of the vehicle.

[0034] Such vehicles are sometimes alternatively referred to as 'micro-hybrid vehicles'.

[0035] The electric motor is not used on its own in a mild hybrid vehicle to drive the vehicle it is only used to start the engine or assist the engine in driving the vehicle so as to reduce the instantaneous fuel consumption of the engine.

[0036] Therefore the BISG 14 can operate in two modes, in the first mode it is driven by the engine 11 to produce electrical power for storage in the high voltage battery 18 (HV battery) and in the second mode is produces torque to either supplement the torque produced by the engine 11 or for use in starting the engine 11.

[0037] The electronic controller is described in this case as being a single control unit 10 operable to control not only the general operation of the engine 11 but also the low and high voltage starter systems. It will however be appreciated that the electronic controller could comprise of a number of interlinked electronic controllers providing in combination the same functionality.

[0038] In the case of the example shown in Fig.1 the inputs to the electronic controller 10 include at least one input from which the temperature of the engine 11 can be deduced and in this case comprise sensor inputs indicative of ambient air temperature; intake air temperature; coolant temperature; cylinder head temperature and engine cylinder block temperature and at least one input indicative of engine speed and/or BISG speed.

[0039] The high voltage battery 18 of the high voltage starter system is operatively connected via the DC to DC converter 19 to the battery 17 of the low voltage starter system so that the low voltage battery can be recharged by the BISG 14 when required.

[0040] The electronic controller 10 is operatively connected to the DC to DC converter 19, to the high and low voltage battery management systems 16 and 15, the starter motor 13 and BISG 14 and various other devices and sensors associated with the engine 11.

[0041] The electronic controller 10 could also include an engine stop-start controller for the motor vehicle 5 and the inputs 20 would then also include inputs for use in determining when the engine 11 should be automatically stopped in order to save fuel. Such a stop is referred to herein as an 'E-stop' because its function is to increase the economy of the engine 11. As is well known in the art various triggers can be used to initiate an E-stop based upon operation of various driver actions and further triggers based upon driver actions can be used to initiate an automatic restart following an E-stop. Any suitable combination of stop and start triggers can be used in accordance with this invention. An automatic engine stop or E-stop is one where the engine 11 is temporarily stopped to save fuel and reduce emissions by the electronic controller 10 in response to one or more conditions based upon driver actions.

[0042] In the case of the example being described 'low voltage' is a voltage of circa 12 volts and 'high voltage' is a voltage of circa 48 volts. It will however be appreciated that the invention is not limited to these voltages although they are advantageous in that equipment utilising such voltages is readily available.

[0043] Operation of the apparatus for starting the engine 11 is as follows.

[0044] When the controller 10 receives an input indicative that the engine 11 is to be started it has to decide whether to use the starter motor 13 to perform the start or the BISG 14. The input indicative that the engine is required to be started could be a manual one resulting from a driver actuation of a starting input device such as a start button or ignition key or can be produced automatically at the end of an "E" stop.

[0045] The default condition for the electronic controller 10 is to use the BISG 14 to start the engine 11 because this will result in a higher start quality and also has the effect of almost instantaneously reducing the state of charge (SOC) of the high voltage battery 18 due to the high current draw required to start the engine 11. Reducing the SOC of the high voltage battery 18 is particularly advantageous if the engine 11 is not up to temperature because it is then desirable to light-off any exhaust aftertreatment devices as soon as possible in order to reduce exhaust emissions and permit the engine control strategy to be more focussed on CO2 reduction.

[0046] By using the BISG 14 to start the engine 11 it is ensured that immediately after the engine 11 has started the BISG 14 can be switched to the first or charging mode to recharge the high voltage battery 18 thereby applying an additional load to the engine 11 which speeds up light-off of any exhaust aftertreatment devices by increasing the temperature of the exhaust gas exiting the engine 11 (exhaust gas heating).

[0047] There are several situations where the starter motor 13 is used to start the engine 11 instead of the BISG 14.

[0048] The first of these is when the engine 11 is sensed to be at a very low temperature well below the normal operating temperature of the engine. At low temperatures the torque required to start the engine 11 increases dramatically, particularly if ice crystals form within the engine 11. Therefore, if the engine temperature is below a minimum temperature (T_min), the starter motor 13 is used to start the engine 11. It will be appreciated that a starter motor 13 is geared so as to produce a large output torque but can only rotate the engine at a relatively low speed. Typically a maximum cranking speed produced by a starter motor will be circa 400 RPM.

[0049] The minimum temperature T_min may be derived from experimental work for each engine/BISG combination but in all cases is the temperature below which the torque available from the BISG 14 is unlikely to be sufficient to produce a clean or good quality start. It will be appreciated that there is a compromise between increasing the torque output capacity of the BISG 14 versus its general operating efficiency and that, at low temperatures, other factors such as drive belt efficiency and durability have to be taken into account. The value of this minimum temperature T_min will however depend upon the size and type of the engine and the torque capacity of the BISG. For example in the case of a small petrol engine it may be possible to use the BISG to produce a clean engine start down to minus 40°C (T_min = -40°C) whereas, in the case of a large diesel engine or large petrol engine, the BISG may not be able to cleanly start the engine below plus 5°C (T_min = +5°C). Therefore T_min is generally within the range of -40°C to +5°C but generally is likely to be a temperature below 0°C in most cases depending upon the above referred to factors and in all case is well below the normal temperature operating range of the engine 11.

[0050] A second situation where the starter motor 13 is used instead of the BISG 14 is when the cranking speed or acceleration of the engine 11 produced by the BISG 14 is unacceptably low.

[0051] In its most simplistic form engine speed can be checked by using a comparison of the current cranking speed Nc with a defined speed limit Nc_min and if the current measure of cranking speed Nc is below the value set for Nc_min reverting to the starter motor 13 to start the engine 11. For example by checking the speed of the engine 11 a predefined period of time after cranking has commenced with an expected engine speed it can be determined if the BISG 14 is able to effectively crank the engine 11. The value of Nc_min can be a variable based upon temperature and/ or time since cranking commenced in which case a look up chart or table referencing time/ temperature and engine speed could be used to check the predicted ability of the BISG 14 to start the engine 11.

[0052] A similar approach could be used if the test is based upon engine acceleration. The engine speed in this case being used to produce a value of engine acceleration which is compared to a predefined or expected rate of acceleration if the BISG is functioning normally and would be expected to produce a good quality start. As before the expected rate of acceleration could be varied based upon engine temperature and a look up chart or table referencing temperature and engine acceleration could be used to check the predicted ability of the BISG 14 to start the engine 11.

[0053] There are various reasons why the cranking speed or rate of engine acceleration could be unacceptably low, for example and without limitation, the resistance to cranking of the engine 11 could be higher than expected, the SOC of the high voltage battery 18 could be low, there could be a fault in the BISG 14, there could be insufficient torque transfer capacity due to drive belt slip or the high voltage battery management system 16 could block the drawing of current from the high voltage battery because the SOC of the high voltage battery is at or below a lower limit.

[0054] A third situation where the starter motor 13 is used instead of the BISG 14 is when the state of charge of a battery used to power the high voltage belt integrated starter-generator is below a predefined level before cranking commences. It will be appreciated that there has to be a certain level of charge in the high voltage battery 18 for it to be able to successfully power the BISG 14 during an engine start. In addition this predefined level of charge will vary depending upon the expected torque required to start the engine 11. Therefore, if the state of charge of the high voltage battery 18 is below the predefined level for the current temperature, it is likely that the BISG 14 will not be able to effectively crank the engine 11 and so the starter motor 13 is used to start the engine 11. However, in most cases the state of charge of the high voltage battery 18 will be sufficient and so this situation is unlikely to arise very often.

[0055] Therefore whenever cranking using the BISG 14 is determined to be unavailable or ineffective the electronic controller 10 is operable to use the starter motor 13 to start the engine 11 even if the temperature is above the low temperature limit.

[0056] It will be appreciated that the function of the high and low voltage battery management systems 16 and 15 is to monitor and control the state of charge of the respective battery 18 and 17 to which they are connected and to prevent over discharging or excessive charging of these batteries 18, 17.

[0057] With reference to Fig.2 there is shown a high level flowchart of a method 100 for starting the engine 11 that could be embodied as software or firmware as part of the electronic controller 10 and forms a first part of a method for improved engine operation in accordance with this invention.

[0058] The method starts in box 110 with a key-on event and with the engine 11 not running, and then advances to box 115 to check whether an engine start request has been received either due to a manual start operation by a user of the engine 11 or due to an automatic start request at the end of an E-stop.

[0059] If no start request has been received the method cycles through box 115 until either a key-off event occurs or an engine start request is received.

[0060] If a key-off event occurs, the method terminates with the engine 11 still not running and, if an engine start request is received, the method advances to box 118.

[0061] In box 118 it is checked whether there is sufficient charge in the high voltage battery 18 to effectively start the engine using the BISG 14 and, if there is not, the method advances to box 160 to start the engine using the starter motor 13 and, if there is sufficient charge in the high voltage battery 18, the method continues to box 120.

[0062] In box 120 it is checked whether a current engine temperature (T) is above a minimum temperature limit (T_min). As previously discussed this temperature limit is a positive temperature within a few degrees of zero degrees Celsius or a negative temperature of up to about minus 20 degrees Celsius. The temperature used for this test could be the temperature of part of the engine 11 such as a cylinder block or cylinder head or the temperature of the engine coolant depending upon the specific arrangement of the engine system and sensors. In one example the temperature limit was set at 0°C and the measured temperature was a measurement of cylinder block temperature.

[0063] The temperature limit (T_min) therefore falls normally within the range of -40 to +5 degrees Celsius depending upon the torque capacity of the BISG 14 and the relationship between the torque required to start the engine 11 and temperature.

[0064] If the current temperature is above the minimum temperature limit T_min then the method advances to box 130 otherwise it advances to box 160.

[0065] In box 130 the engine 11 is cranked using the BISG 14 and the resulting cranking speed (Nc) is measured using a rotational speed sensor. Then in box 140 it is checked whether the cranking speed is above a speed limit Nc_min chosen to represent a speed where a good quality engine start will be produced if the BISG 14 is operating correctly. The speed is not necessarily the final speed expected to be reached by the engine 11 when cranked by the BISG 14 it could be a speed expected to be attained by the engine a predetermined period of time after initiation of cranking. This test is used to confirm that the BISG 14 is operating correctly and is capable of producing a high quality engine start. As previously referred to, this test could be replaced by an engine acceleration test. In which case the test would be replaced in box 140 with:-

[0066] Is the measured engine acceleration greater than a required rate of engine acceleration?

[0067] It will be appreciated that the logic used could be reversed so that the test could check whether the engine speed or engine acceleration is less than a defined value and in which case the Values of "Yes" and "No" would need to be reversed.

[0068] However, as shown, if Nc is greater than Nc_min then the method advances to box 150 otherwise it advances to box 160.

[0069] In box 150 it is checked whether the engine 11 has started. This check could be performed by measuring the rotational speed of the engine or by any other means such as for example measuring fuel usage or monitoring exhaust gas constituents.

[0070] If the engine 11 has started when checked in box 150 the engine start method advances to box 190 with the engine running otherwise it returns from box 150 to box 130 to continue cranking the engine 11 using the BISG 14 and cycles through boxes 130 to 150 until the engine 11 has started.

[0071] A further step may be included in the return from box 150 to box 130 to check that the current SOC of the high voltage battery 18 is above a predefined lower charge limit and if it is not starting using the BISG 14 will need to be aborted.

[0072] Referring now back to boxes 120 and 140, if these tests are failed the method advances in both cases to box 160 where the engine 11 is cranked using the starter motor 13 and then advances to box 170 to check whether the engine 11 has started. If the engine has not started the method cycles through boxes 160 and 170 until it does start and when it does start advances to box 190 where the engine start method ends with the engine 11 running.

[0073] It will be appreciated that the method 100 will be ended at any step if a key-off event occurs. It will also be appreciated that, if the engine has not started within a predefined period of time following initiation of cranking, further steps (not shown) may be provided to halt cranking of the engine to prevent damaging either the starter motor 13 or the BISG 14 depending upon which starting device is being used.

[0074] Fig.3 shows a flow chart of a method 200 forming a second part of the method for improved engine operation in accordance with the first aspect of the invention.

[0075] The method uses the engine starting method 100 shown in Fig.2 and follows on from step 190 of that method with the engine running.

[0076] In box 210 it is checked whether the temperature of the exhaust gas from the engine 11 needs to be increased (exhaust gas heating) in order to assist with light-off of any exhaust aftertreatment devices. This step could be carried out by means of a comparison of a current measurement of exhaust temperature using a temperature sensor located in or adjacent to each aftertreatment device 6 with a predefined temperature limit or by any other convenient means including exhaust temperature modelling.

[0077] One example of the test in box 210 is:-

Is Texh < Texh_lim ? If yes GoTo 220 else GoTo 260

[0078] Where:-

Texh is the current measured exhaust gas temperature; and

Texh_lim is an exhaust gas temperature above which light-off of the respective aftertreatment device 6 can be expected to have occurred.

[0079] Therefore, assuming that exhaust gas heating is required, the method advances from box 210 to box 220.

[0080] In box 220 it is checked whether the HV battery 18 can be charged. Normally if the BISG 14 has been used to restart or start the engine 11 then there will be sufficient headroom (the difference between current SOC and maximum permitted SOC) in the high voltage battery 18 (HV battery 18) to permit it to be recharged. However, if the starter motor 13 has been used to start the engine 11, then the SOC of the HV battery 18 could be too high to permit charging to be carried out.

[0081] That is to say, the SOC of the HV battery 18 could be at the maximum charging limit (fully charged) or so close to this limit that the amount of charging that could be carried out would have no real effect on exhaust gas heating.

[0082] If charging is not possible the method in this case advances to box 290 where it ends. After reaching box 290 control of the BISG 14 will then revert to normal control to meet any needs of the HV battery 18 and low voltage battery 17 as determined by the high and low voltage battery management systems 16 and 15 and/ or torque assist of the engine 11 if such a facility is permitted and is required.

[0083] Torque assist is the use of the BISG 14 to provide torque to assist the engine 11. That is to say for a given torque requirement some of the torque is provided by the BISG 14 thereby reducing the amount of torque required from the engine 11 and resulting in improved fuel economy.

[0084] It will be appreciated that the BISG 14 could also or alternatively be used for 'torque boosting'. Torque boosting is where the BISG 14 is used to increase the torque supplied to the vehicle above that available solely from the engine 11. This will not reduce the fuel consumption of the engine 11 but instead will increase vehicle performance.

[0085] If in box 220 it is confirmed that there is sufficient headroom in the HV battery 18 for charging to occur then the method advances from box 220 to box 230 and the BISG 14 is operated in the first mode as a generator to load the engine 11 thereby increasing the exhaust gas temperature exiting the engine 11. In such a case the BISG 14 is operated at or close to its maximum output level in order to produce the maximum loading on the engine 11.

[0086] From box 230 the method advances to box 240 which in practice occurs simultaneously with the action performed in box 230. That is to say, the SOC of the HV battery 18 is continuously monitored to prevent overcharging of the HV battery 18. If the SOC of the HV battery 18 is below the maximum charging limit then charging can continue but it is first checked whether exhaust gas heating is still required and so the method returns to box 210. The method will then proceed through boxes 220 and 230 back to box 240 provided heating is still required and will continue to cycle through boxes 210 to 240 until either the maximum charging limit of the HV battery 18 is reached or heating of the exhaust gasses is no longer required.

[0087] If the maximum charging limit is reached then the method advances from box 240 to box 250 where charging of the HV battery by the BISG 14 is terminated and will then advance to box 290 where it will end and, as previously described, the control of the BISG 14 will revert to normal control to meet any needs of the HV battery 18.

[0088] If exhaust gas heating is no longer required the method will advance from box 210 to box 260.

[0089] If torque assist or torque boosting is not a function available on the vehicle 5 then the method would instead advance directly from box 210 to box 290 and boxes 260 to 280 would not be present.

[0090] In box 260 it is checked whether torque assist is required and, if it is not, the method advances from box 260 to box 290 where it ends. It will be appreciated that torque assist is not permitted if the current exhaust gas temperature is below the exhaust gas temperature limit Texh_lim below which light-off of the respective aftertreatment can be expected not to have occurred. This is because torque assist will have the effect of reducing exhaust gas temperature not increasing it and so will delay light-off of the aftertreatment devices 6.

[0091] If torque assist is required and permitted, the method advances from box 260 to box 270 where the current SOC of the HV battery 18 is checked. The SOC of the HV battery 18 is not allowed to fall below a lower charge limit and so before permitting torque assist is allowed it is checked to ensure that a sufficient amount of charge is present in the HV battery 18 to produce a worthwhile torque assist effect.

[0092] If there is insufficient charge in the HV battery 18 to permit torque assist, the method advances from box 270 to box 290 where it ends. However, if in box 270 it is confirmed that sufficient charge is available to permit torque assist, the method advances from box 270 to box 280 where the BISG 14 is operated as a motor to provide assistance to the engine 11.

[0093] From box 280 the method returns to box 260 and will then continue to cycle through boxes 260, 270 and 280 until either the SOC of the HV battery 18 falls to a level below which torque assist is not permitted (box 270 fail) or torque assist is no longer required (box 260 fail). In either of these cases the method will advance from the respective box to box 290 where it ends and, as previously described, the control of the BISG 14 will then revert to normal control to meet any needs of the HV battery 18.

[0094] Therefore in summary the belt integrated starter motor is preferably used to start the engine because this will result in a discharging of the associated high voltage battery which can then be rapidly recharged following an engine start to aid with heating of the exhaust gasses and reduce the time required for any exhaust aftertreatment device to reach their respective light-off temperatures. The starter motor is only used if the BISG is not capable of producing an effective start or the temperature is so low that use of the BISG is not possible.

[0095] By giving preference to starting of the engine using the BISG whenever possible ensures that the BISG can be used, if required, immediately following the start as a generator load for the engine so as to assist with exhaust gas heating.

[0096] Some advantages of the invention are:

a/ Conducting an engine start using the 48V BISG, consumes energy from the 48V battery and then immediately after cranking ends the 48V BISG can be used to re-charge the 48V battery and so load the engine. This engine loading will generate more exhaust heat during a cold start period following start-up and will result in faster exhaust gas aftertreatment light-off than would otherwise be possible;

b/ The faster light-off will result in lower engine emissions which will allow the use of a Mild Hybrid control strategy that is more focussed on CO2 reduction without exceeding emission targets for the engine;

c/ When compared to the cranking attributes of a 12V starter, the driver in the vast majority of cases will get a higher start quality for the first engine start due to the use of the 48V BISG. The 12V starter is therefore only used when the ambient temperature is low such as less than about 4°C or 48V BISG system is unable to crank effectively;

d/ Using the BISG to start the engine produces a better quality start in terms of noise and vibration than a starter motor and less mechanical wear in terms of the components required to effect the start; and

e/ Providing a larger range of operation for the BISG increases the possibilities for stop-start operation. That is to say, E-stop can be performed even if the engine has not reached it normal operating temperature range.

[0097] It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more embodiments it is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for improved engine operation comprising starting an engine (11) of a motor vehicle (5) having a high voltage belt integrated starter-generator (14) and a low voltage starter motor (13) characterised in that the high voltage belt integrated starter-generator (14) is used to crank the engine (11) whenever it is able to effectively crank the engine (11) and, if after starting of the engine (11), it is required to light-off one or more exhaust aftertreatment devices (6), using the high voltage belt integrated starter-generator (14) as a generator so as to load the engine (11) to increase the temperature of the exhaust gas from the engine (11).

2. A method as claimed in claim 1 wherein the method further comprises using the high voltage belt integrated starter-generator (14) as a generator to increase the temperature of the exhaust gas of the engine (11) until it is no longer required to further increase the temperature of the exhaust gas.

3. A method as claimed in claim 1 or in claim 2 wherein the method further comprises using the high voltage belt integrated starter-generator (14) as a generator only if the state of charge of a battery (18) charged by the high voltage belt integrated starter-generator (14) is below a predefined upper charging limit.

4. A method as claimed in any of claims 1 to 3 wherein, if after starting of the engine (11) and provided there is no requirement to light-off one or more exhaust aftertreatment devices (6), the method further comprises determining whether torque assist of the engine (11) is required and, if torque assist is required, using the high voltage belt integrated starter-generator (14) as a motor to provide torque assist to the engine (11).

5. A method as claimed in claim 4 wherein the method further comprises using the high voltage belt integrated starter-generator (14) for torque assist only if the state of charge of a battery (18) providing power to the high voltage belt integrated starter-generator (14) to operate it as a motor is above a minimum permitted charge level.

6. A method as claimed in any of claims 1 to 5 wherein the high voltage belt integrated starter-generator (14) is able to effectively crank the engine (11) unless the temperature (T) of the engine (11) is below a predefined low temperature limit (T_min) below which the engine (11) is started using the low voltage starter motor (13).

7. A method as claimed in any of claims 1 to 6 wherein the low temperature limit (T_min) is a temperature well below the normal operating range of the engine (11).

8. A method as claimed in claim 6 or in claim 7 wherein the low temperature limit (T_min) is a temperature below zero degrees Celsius.

9. A method as claimed in claim 6 or in claim 7 wherein the low temperature limit (T_min) is a temperature falling within a range of minus 40 to plus 5 degrees Celsius.

10. A method as claimed in any of claims 1 to 9 wherein the high voltage belt integrated starter-generator (14) is able to effectively crank the engine (11) if it is able to crank the engine (11) at a speed (Nc) greater than a defined rotational speed (Nc_min).

11. A method as claimed in any of claims 1 to 9 wherein the high voltage belt integrated starter-generator (14)) is able to effectively crank the engine (11) if it is able to produce an acceleration of the engine (11) greater than a defined acceleration.

12. A method as claimed in any of claims 1 to 11 wherein the high voltage belt integrated starter-generator (14) is able to effectively crank the engine (11) if the state of charge of a battery (18) used to power the high voltage belt integrated starter-generator (14) is above a predefined level before cranking commences.

13. A mild hybrid motor vehicle having an apparatus for improved engine operation comprising a high voltage belt integrated starter-generator (14), a low voltage starter motor (13) and an electronic controller (10) to control the operation of at least the high voltage belt integrated starter-generator (14) and the low voltage starter motor (13) characterised in that the electronic controller (10) is operable to use the high voltage belt integrated starter-generator (14) to crank the engine (11) whenever it is able to effectively crank the engine (11) and, if after starting of the engine (11), it is required to light-off one or more exhaust aftertreatment devices (6) of the motor vehicle (5), use the high voltage belt integrated starter-generator (14) as a generator so as to load the engine (11) to increase the temperature of the exhaust gas from the engine (11).

14. A motor vehicle as claimed in claim 13 wherein the high voltage belt integrated starter-generator (14) is able to effectively crank the engine (11) unless the temperature (T) of the engine (11) is below a predefined low temperature limit (T_min) below which the engine (11) is started using the low voltage starter motor (13).

Drawing